Question: Gabriela is 2 times as old as Jessica. 30 years ago, Gabriela was 7 times as old as Jessica. How old is Gabriela now?
Explanation: We can use the given information to write down two equations that describe the ages of Gabriela and Jessica. Let Gabriela's current age be $g$ and Jessica's current age be $j$ The information in the first sentence can be expressed in the following equation: $g = 2j$ 30 years ago, Gabriela was $g - 30$ years old, and Jessica was $j - 30$ years old. The information in the second sentence can be expressed in the following equation: $g - 30 = 7(j - 30)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $g$ , it might be easiest to solve our first equation for $j$ and substitute it into our second equation. Solving our first equation for $j$ , we get: $j = g / 2$ . Substituting this into our second equation, we get: $g - 30 = 7($ $(g / 2)$ $- 30)$ which combines the information about $g$ from both of our original equations. Simplifying the right side of this equation, we get: $g - 30 = \dfrac{7}{2} g - 210$ Solving for $g$ , we get: $\dfrac{5}{2} g = 180$ $g = \dfrac{2}{5} \cdot 180 = 72$.